The invention relates generally to information storage devices. More particularly it relates to an improved servowriting system and method for implementation in a storage device wherein errors introduced into the written position or servo information are reduced or eliminated.
Increased levels of storage capacity in storage devices such as removable disk disk-drives, hard disk drives, and tape drives are a direct result of the higher track densities possible with voice-coil and other types of servo positioners as well as the ability to read and write narrower tracks by using, for example, magnetoresistive (MR) head technology. Head positioning is accurately controlled using positional information stored on the disk itself, as in dedicated and embedded servo architectures.
Conventional servo-patterns in an xe2x80x9cembedded servoxe2x80x9d disk drive architecture typically comprise short bursts of a constant frequency signal, very precisely offset to either side of a data track""s center line. The bursts precede data regions of the track and are used to align a head with respect to the track center. Staying on track center is required during both reading and writing for accurate data storage and retrieval. Since there can be, for example, sixty or more data regions per track, that same number of servo data areas are preferably distributed around a track to provide means for a head to follow the track""s center line as the disk rotates, even when the track is out of round, e.g., as a result of spindle wobble, disk slip and/or thermal expansion. As technology advances to provide smaller disk drives and increased track densities, the accurate placement of servo data must also increase proportionately.
Servo-data are conventionally written by costly dedicated servowriting equipment external to the disk drive equipped with large granite blocks for supporting the drive and quieting external vibrational effects. An auxiliary clock head is inserted onto the surface of the recording disk to write a reference timing pattern, and an external head/arm assembly is used to precisely position the transducer. The positioner includes a very accurate lead screw and a laser displacement measurement device for positional feedback. Servotracks are written on the media of a head disk assembly (HDA) with a specialized servowriter instrument. Laser positioning feedback is used in such instruments to read the actual physical position of a recording head used to write the servotracks.
A disadvantage of servo writers such as those described is that they require a clean room environment, as the disk and heads will be exposed to the environment to allow the access of the external head and actuator. Additionally, it is becoming more and more difficult for such servowriters to invade the internal environment of an HDA for servowriting because the HDAs themselves are exceedingly small and depend on their covers and castings to be in place for proper operation. Some HDAs, for instance, are the size and thickness of a plastic credit card.
In view of these challenges, a disk drive able to perform self-servo writing would be tremendously advantageous. However, this approach presents a new set of challenges. Specifically, self-servowriting systems are more prone to mechanical disturbances. Moreover, because of the interdependency of propagation tracks in self-servowriting, track shape errors introduced by mechanical disturbances and other factors may be amplified from one track to the next when writing the propagation tracks.
Servopatterns consist of bursts of transitions located at intervals around the disk surface. In self-propagation, the radial position signal that is used to servo-control the actuator is derived from measurements of the readback amplitude of patterns that were written during a previous step of the servowrite process. That is, the burst edges of a written track comprise a set of points defining a track shape that the servo controller will attempt to follow when writing the next track. Thus, errors in the transducer position during burst writing appear as distortions away from a desired circular track shape. The servo controller causes the actuator to follow the resulting non-circular trajectory in a next burst writing step, so that the new bursts are written at locations reflecting (via the closed-loop response of the servo loop) the errors present in the preceding step, as well as in the present step. Consequently, each step in the process carries a xe2x80x9cmemoryxe2x80x9d of all preceding track shape errors. This xe2x80x9cmemoryxe2x80x9d depends on the particular closed-loop response of the servo loop.
A primary requirement in disk files is that each track be separated at all points by some minimum spacing from adjacent tracks. This requirement ensures that neighboring track information is not detected on readback, resulting in data read errors, and more importantly, that adjacent track data will never be excessively overlapped during writing, potentially resulting in permanent loss of user data. In other words, the detailed shape of each track relative to its neighbors must be considered, not just the track to track distance averaged around the whole disk. Radial separation between adjacent track locations is determined by the product servopattern written on each track and at each angular location around the disk, since the servo-control of the actuator during actual file operation is capable of following distortions from perfect circularity and will produce misshapen data tracks.
A consideration in setting the minimum allowable spacing for self propagation schemes is the existence of random fluctuations about the desired track location resulting from mechanical disturbances during actual file operation. Effects that result in track shape errors in a self-servowriting system include, for example, include random mechanical motion and modulation in the width of the written track that results, e.g., from variations in the properties of the recording medium or the flying height of the transducer. Random mechanical motion can be lowered using a high gain servo loop, but this leads to error compounding. Random magnetic modulation variations reduce the pattern accuracy of all servowriters, but error compounding in self-propagation can further amplify its effects. Uncontrolled growth of such errors can lead to excessive track non-circularity, and in some cases, may even lead to exponential growth of errors, exceeding all error margins and causing the self-propagation process to fail.
One of the largest sources of mechanical disturbance in a disk drive is the turbulent wind generated by the spinning disks blowing against the actuator. The total amount of fluctuation, referred to as track misregistration or TMR, defines a relevant scale for judging the required accuracy of servopattern placement. If servopattern placement errors are roughly equal to or greater than the TMR, then a substantial fraction of the track spacing margin will be required as compensation and the total disk file data capacity will accordingly be reduced. In this instance, reduction in servopattern errors may substantially increase file capacity.
Consequently, self-servowriting systems must provide a means for accurately writing servopatterns while controlling the propagation of track shape errors.
One self-servo writing method is disclosed in U.S. Pat. No. 4,414,589 to Oliver et al., which teaches optimization of track spacing. Head positioning is achieved in the following manner. First, one of the moving read/write heads is positioned at a first stop limit in the range of movement of the positioning means. The head is used to write a first reference track. A predetermined percentage of amplitude reduction, X%, is selected that empirically corresponds to the desired average track density. The moving head reads the first reference track and is displaced away from the first stop limit until the amplitude of the signal from the first reference track is reduced to X% of its original amplitude. A second reference track is then written by the head at the new location, read, and the head is again displaced in the same direction until the amplitude of signal from the second reference track is reduced to X% of its original value. The process is continued until the disc is filled with reference tracks. The average track density is checked to insure that it is within a predetermined acceptable range of the desired average track density. If the average track density is too high or too low, the disk is erased, the X% value is appropriately lowered or increased, and the process is repeated. If the average track density is within the predetermined acceptable range, the desired reduction rate X% for a given average track density has been determined and the servo writer may then proceed to the servo writing steps.
U.S. Pat. No. 4,912,576 to Janz and U.S. Pat. No. 5,448,429 to Cribbs et al. describe methods for writing a servo-pattern with a disk drive""s own pair of transducers. Three types of servo-patterns are used to generate three-phase signals that provide a difference signal having a slope directly proportional to velocity. Janz observes that the signal level from a transducer is a measure of its alignment with a particular pattern recorded on the disk. For example, if the flux gap sweeps only forty percent of a pattern, then the read voltage will be forty percent of the voltage maximum obtainable when the transducer is aligned dead-center with the pattern. Janz uses this phenomenon to position the heads by straddling two of three offset and staggered patterns along a centerline path intended for data tracks. In a preferred process, Janz describes a dedicated servo architecture wherein one side of a disk is reserved for servo and the other side for data. The disk drive includes two transducers on opposite surfaces that share a common actuator. To format an erased disk for data initialization, a first phase servo is written on the servo side at an outer edge. The transducers are then moved-in radially one half of a track, as indicated by the first phase servotrack amplitude, and a first data-track is recorded on the data side. The transducers are again moved radially inward one half of a track, this time as indicated by the first data-track amplitude, and a second phase servotrack is recorded on the servo side. This sequence is repeated until both surfaces are entirely written. If too few or too many tracks are written, the disk is reformatted with a slight adjustment to the step width, as determined by the track count. Once the disk drive has been formatted with an entire compliment of properly spaced servotracks, the data-tracks are erased in preparation for receiving user data. Unfortunately, the method described by Janz requires a dedicated disk surface for servotracks and two heads working in tandem. Moreover, transducer flying height variations, spindle runout, and media inconsistencies can corrupt radial position determinations that rely on a simple reading of off-track read signal amplitudes. Prior art methods do not address these issues and are therefore inadequate for high performance disk drives applications.
Cribbs et al. teaches a hard disk drive system with self-servowriting capability comprising a rotating recording disk, transducer in communication with the disk surface, servo-actuator means for radially sweeping the transducer over the surface, a variable gain read amplifier (VGA) coupled to the transducer, an analog to digital converter (ADC) coupled to the VGA, an erase frequency oscillator coupled to the transducer for DC erasing of the disk surface, a memory for storing digital outputs appearing at the ADC, and a controller for signaling the servo-actuator to move to such radial positions that result in transducer read amplitudes that are a percentage of previous read amplitudes represent in the digital memory.
The prior art fails to solve the problem of random mechanical motion resulting in reduced disk file capacity, or to address the relationship between random mechanical motion and error compounding. It also fails to address the relationship between magnetic write width modulation and error compounding, or even error compounding viewed independently.
Accordingly, what is needed is a self-servowriting system and method which overcomes the preceding problems to provide a highly accurate and cost effective positioning system applicable to not only hard disk drives, but also to other storage devices wherein head positioning is achieved using similar principles.
It is the principal object of this invention to provide an improved method and system by which errors introduced into position or servo information can be kept to a minimum in the servowriting process.
Accordingly, the present invention is a servowriting method and system having one or more of the following improved features: 1) optimized transducer write current and frequency for minimized write width modulation; 2) optimized servowriting RPM for minimized random mechanical vibration; and 3) clip level checking to eliminate the introduction of large errors in product servopattern.